Accessory films featuring closely spaced louvers have already been used for mobile displays to provide privacy of the optical data displayed. Rather than being switchable, though, these films have to be applied and removed manually, and they have to be carried separately from the display when not in use. Another essential disadvantage of such louver films is the light loss they entail.
Such a privacy filter using closely spaced louvers is described in U.S. Pat. No. 6,765,550. Here, the troublesome disadvantages are the need to remove or attach the filter mechanically, and the light loss in the protected mode.
U.S. Pat. No. 5,993,940 describes the use of a film the surface of which is provided with a regular arrangement of narrow prismatic stripes to achieve a privacy mode. The development and fabrication of this film are rather complicated.
In WO 2012/033583, switching between free and restricted view is brought about by triggering liquid crystals between layers known as “chromonic”. This entails a light loss, and the technology is quite complex.
US 2009/0067156 discloses quite a number of ideas of how to configure a lighting system and a display unit. The version illustrated by FIGS. 3A and 3B therein uses, in particular, two backlights consisting of wedge-shaped light guides, and an LCD panel, with the rear backlight 40 being claimed to compellingly generate a wide illuminating angle and the backlight 38 in front of it being claimed to compellingly generate a narrow illuminating angle. It is not clear, though, by what mode of operation the backlight 38 is meant to generate a narrow illuminating angle without converting the light coming from backlight 40 with a wide illuminating angle into light with an essentially narrow illuminating angle when it passes backlight 38.
Concerning the configuration according to FIG. 5 of US 2009/0067156, it is to be remarked that light guides 46 and 48 both produce “narrow light”, i.e. light with a narrow illuminating angle. Conversion of the light of light guide 48 into “wide light”, i.e. light with a wide illuminating angle, is brought about only by means of a beam-splitting mirror 50 with a prism structure that is complicated to fabricate. This conversion extremely curtails the light intensity, since the light first radiated in a narrow angle, which is the only light available, is then fanned out over a large angle, which is the half-space, as a rule. As a result, brightness (in terms of luminance) is diminished by a factor of 5 or more, depending on the parameters. Thus, this is a configuration of little relevance in practice.
In the configuration according to FIG. 7 of US 2009/0067156, a layer of phosphor is essential to convert UV light into visible light. This is quite a complex design, and given the need to get sufficient light from the backlight to illuminate an LCD panel so as to permit reading it, UV light of very high intensity is required. This is expensive and complicated; shielding the necessary UV radiation alone makes this configuration impracticable.
US 2012/0235891 describes a highly complex backlight in a display screen. According to FIGS. 1 and 15 there, not only several light guides are employed, but also further complex optical elements such as microlens elements 40 and prism structures 50, which convert the light emitted by the rear backlighting system on its way to the second backlighting system. To implement this is expensive and complicated, and it entails light loss as well. In the version according to FIG. 17 in US 2012/0235891, light sources 4R and 18 both produce light with a narrow illuminating angle, with the light emitted by the rear light source 18 first being converted into light with a large illuminating angle in a complex manner. As mentioned above, this complex conversion diminishes brightness significantly.
In JP 2007-155783, special optical surfaces 19 that are complicated to design and fabricate are used to deflect light into various narrow or wide ranges depending on the angle of light incidence. These structures resemble Fresnel lenses and feature inactive edges that deflect light into unwanted directions. Thus, it remains unclear whether really useful light distributions can be achieved.
As a rule, the methods and arrangements mentioned above have the following disadvantages in common: They distinctly reduce the brightness of the basic display screen, and/or require an active optical element for switching between modes, and/or are complicated and expensive to fabricate, and/or reduce resolution in the free viewing mode.